1. Field of the Invention
The present invention is generally related to a sensor and, more particularly, to a method for manufacturing a proximity sensor that is efficient and results in a proximity sensor which is able to withstand rigorous use.
2. Description of the Prior Art
Many different types of sensors are known to those skilled in the art. One particular type of sensor is a proximity sensor that utilizes a core and coil assembly associated with an electronic circuit for the purpose of detecting the presence or absence of an electrically conductive object within its detection zone. Proximity sensors that are designed to respond to all types of metals within their detection zone are typically radio frequency inductive devices that are operated on the Eddy Current Killed Oscillator (ECKO) principle. The oscillator consists of an LC tank circuit and an amplifier circuit which has positive feedback. The oscillator frequency is determined by the inductance and capacitance of the LC network. The inductance portion of the tuned circuit is formed by the sensor coil and a ferrite core. The oscillator circuit has just enough positive feedback to keep it oscillating. This oscillation generates an AC waveform which varies in amplitude depending on whether or not a conductive target is present within the detection zone.
Known manufacturing techniques for making a proximity sensor typically mount a plurality of electronic components on a preformed printed circuit board and then attach a core and coil assembly to one end of the printed circuit board and a connector assembly to the opposite end of the printed circuit board. This assembled structure is then inserted into a tubular housing. The remaining spaces within the tubular housing that surround the components of the assembled structure are filled with a curable epoxy. The epoxy is typically heat curable. When the epoxy hardens, the proximity sensor comprises a tubular housing filled with a solid mass of epoxy which surrounds the printed circuit board, the core, the coil, the connector and the electronic components attached to the printed circuit board.
The epoxy used in the manufacturing method described above serves the purpose of preventing the electronic components of the proximity sensor from being damaged by impact forces imposed on the sensor. In addition, the epoxy provides protective sealing in harsh environments, provides thermal dissipation from the electronic components of the proximity sensor and isolates the electronic circuitry within the sensor.
The process described above increases the cost of the proximity sensor by requiring the steps of injecting the liquid epoxy into the housing, assuring that the epoxy fills the entire region within the housing surrounding the components of the sensor and then waiting the required time until the epoxy is completely cured so that the individual sensors can be packaged for shipment. The curing process for a typical epoxy, such as amine cured resins, usually takes about four to eight hours at an elevated temperature for complete curing to take place.
U.S. Pat. No. 5,121,289, which issued to Gagliardi on Jun. 9, 1992, discloses an encapsulatable sensor assembly that includes an external housing with internal support components for retaining an active sensor element and associated electrical circuit in a predetermined relationship while providing a plurality of internal sequentially interconnected cavities for facilitating a flow of an encapsulating material from an encapsulating material injection port to a vented overflow to assure a complete fill of the internal cavities to restrain the internal components. The sensor assembly provides a structure for a void free encapsulation and internal lead wire stress relief concurrently with location control of the internal components while also providing a combination which is easily adaptable to automatic assembly and encapsulation techniques.
Some proximity sensor use individual rigid printed circuit boards to support the electronic components, the core, the coil and the connector. Other types of magnetically sensitive devices have used flexible circuits for these purposes. Rather than using a rigid printed circuit board, these devices incorporate a flexible material that provides the electrically conductive connections and can be used to support a plurality of electronic components thereon.
U.S. Pat. No. 5,349,500, which issued to Casson et al on Sep. 20, 1994, discloses a direct application of an integrated circuit to a flexible printed circuit. The apparatus can be used for electrically connecting flip chips to a flexible printed circuit substrate. A method comprises providing solder paste to a plurality of active contact pads located on the flexible printed circuit substrate and then placing the flip chips on the substrate such that solder bumps located on the flip chips are in registration with the solder paste on the active contact pads. Thirdly, the method includes the step of heating the resulting assembly as a whole so that the solder past on each active contact pad flows to form an electrical connection with its corresponding solder bump.
U.S. Pat. No. 5,364,707, which issued to Swisher on Nov. 15, 1994, describes a metal film laminate resistant to delamination. The invention can comprise a layered film structure having a metal layer securely bonded to a film layer. The laminate contains a unique metal oxide attachment structure between the film and metal layer comprising randomly distributed regions of metal oxide. The peel strength of such a laminate is significantly improved over prior laminates and is resistance to peel strength reduction due to environmental stress. The preferred metal film laminates made with polyester or polyamide can be used in the manufacture of high quality, low costs, flexible printed circuit boards.
The manufacturing costs of proximity sensors made in accordance with known methods can be excessive because of the manual assembly steps that are typically required and, in addition, as a result of the difficult and time consuming use of liquid epoxy which must be inserted into each housing individually and then allowed to cure over a relatively long period of time. It would therefore be significantly beneficial if a means were provided to manufacture proximity sensors in a more efficient way that did not require the individual insertion of liquid epoxy into individual housings of the proximity sensors and the time consuming curing cycle to allow the liquid epoxy to properly cure.